Sure Monahan can score, but that is far from all he does. For one, he can do it from anywhere up and down the FWD lineup, either at wing or C. I've even seen him have success with friggin Pearson and Gallagher for a while (my friends called them the MPG line, wondering how many miles P and G have in the tank). He is rocking it in the faceoff circle and is always where he's supposed to be on the ice.
He's also been taking on defensive missions (PK, D Zone faceoffs). Though Lekhonen kind of had a silent "working hard ethic" type of leadership, Monahan strikes me as being much more of a leader and a voice in the locker room.
While RFA may certainly be worth something, I think that having a guy that can offer you 2nd line C scoring, can be put in any situation (W or C), has a positive voice in your lineup and has talent is definitely worth more than a bottom 9 W that excels in D situations. I also think that getting Monahan for what's left of a 1.9 M contract that expires at then end of the season is a major plus and negates at least part of the risks of him getting injured before the end of the season.